This invention relates to a stabilizer for use in a halogen-containing resin, and more particularly to a stabilizer for a halogen-containing resin which is of substantially reduced toxicity and has excellent heat stabilization properties.
In general, a halogen-containing synthetic resin, when heat-molded, is easily subject to heat-decomposition, mainly resulting from dehydrohalogenation, or, when practically used, it similarly tends to decompose by exposure to sunlight. As a result, it gradually becomes discolored, e.g., yellowish orange, pink, and/or brown, and at the same time its physical properties and durability deteriorate. In order to inhibit such deterioration, therefore, particularly in heat mold processing, a number of stabilizers have been proposed and used. Typical examples are a lead-based stabilizer, a metal soap-based stabilizer, an organic tin-based stabilizer, etc.
These stabilizers provide satisfactory heat resistance, but they often suffer from disadvantages in that halogen-containing synthetic resins containing such stabilizers are inferior in sulfide resistance, lack transparency, are somewhat toxic and are expensive. In many cases, therefore, they are used in combination with each other to partially compensate for their disadvantages. Recently, in particular, mold processing has often been carried out at a high temperature range and therefore it has increasingly been desired to develop or discover those stabilizers which provide heat stability at such high temperature molding and at the same time which are of as low toxicity as possible at the time of operation and use thereof.
In view of the circumstances described above, various inorganic stabilizers have been proposed, and zeolite is known as an inorganic stabilizer. For example, U.S. Pat. No. 3,245,946 discloses a molecular sieve zeolite A which is activated by dehydration at a temperature of at least 300.degree. C.; U.S. Pat. No. 4,000,100 discloses use of an inactive zeolite having a water content of from 18 to 25% in combination with another stabilizer; and Japanese Patent Application (OPI) No. 34356/79 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses the use of an aluminosilicate of zeolite crystal having an ion (sodium salt) exchange capacity of 2.1 m.eq./g.
In all the patents as described above, sodium zeolite A containing Na.sup..sym. in the crystal structure is mainly used. Although substituted zeolite such as calcium zeolite A is suggested to have an effect similar to that of sodium zeolite A, no concrete examples are disclosed.
However, the inventors' extensive experiments on the heat stabilization ability of zeolite on the halogen-containing resin, particularly a vinyl chloride resin, have revealed that when the sodium zeolite is used, coloration showing a reddish tint characteristic of sodium occurs from the beginning of heating, and thus it is not practical to use sodium zeolite as a stabilizer. As a result of further investigations, it has been found that by ion-exchange of the sodium with zinc, zeolites can be obtained an excellent heat stabilization ability and provide excellent weather resistance and moldability (Japanese Patent Application No. 70649/79, filed June 7, 1979).
These metal substituted zeolites, however, still have disadvantages. For example, their effects greatly vary when they are incorporated in the halogen-containing synthetic resin and, in particular, a great difference in the weather resistance of the resin occurs.